I have a two wire system and after installing GFCI outlet in kitchen I realized there is a problem with some type of inconsistancy causing the GFCI to trip as soon as power is sent to it. Thinking the problem was in an older GFCI outlet I attemped to reuse I purchased and installed a GFCI breaker. Same problem. I'm thinking there is possibly a shared ground in the wiring as the kitchen, dining room and basement lights are all on this breaker. In previous testing while installing a new light I noticed some low voltage current coming through even when the switch was off. Is it possible to get a GFCI to work under these circumstances? There is one outlet downstream from the GFIC outlet. I installed a new conventional outlet in place of the GFCI after installing the GFCI breaker. Everything appeared to work fine with a 20 amp breaker and no GFCI outlet installed.

3 Answers
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Sounds like the circuit is sharing a neutral with another circuit. With your GFCI breaker installed, turn off all other breakers. Reset the GFCI breaker, it should now hold. If it does not hold, then you have a problem with the wiring on that circuit and need to open each box to investigate. If the GFCI breaker holds now, turn each breaker on one at a time until you find the circuit that trips the GFCI. This is the circuit with a shared neutral. You then need to open switch boxes, light boxes, on both circuits to figure where the neutrals were wrongly tied together. Suspect anything that looks like a homeowner addition, or fix. A common place this might happen would be in a switch box that controls different lights, with two or more circuits feeding the box. An unknowing DYI -er may have tied all neutrals together.

To narrow the search, if the GFCI outlet had the same issue, then the problem is likely on the "load" side of the old outlet. Check any downstream junction boxes that lost power when the GFCI outlet tripped.
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BMitch♦Mar 18 '12 at 11:39

I had this issue as well. It turned out that someone tied the neutral from a separate circuit into the the GFCI protected lighting circuit. They needed a 3 wire to make the light switch work properly. Not having a three wire, they used the neutral wire as the hot from the switch. Not having a neutral anymore they ran a short wire from the closest light to provide a neutral return, being fed from a different breaker. It took 3 hours of troubleshooting to figure this one out.